A surreptitious inclusion of code words or phrases that will be heard by some of those listening, while not disturbing the other listeners, who may not appreciate the hidden message(s).

President George W. Bush is constantly littering his speeches with dog whistle comments meant for the religious right, so that he can tell them what they wants to hear, while not offending those who aren't evangelical or born again. (paraphrase of text by Ian Welsh, on his Agonist blog)